US2016222A - Catalytic production of sulphur trioxide - Google Patents

Description

Oct. 1, 1935. P BASSETT 2,016,222

CATALYTIC PRODUCTION OF SULPHUR TRIOXIDE Filed March 1, 1952 g 50%; Kama @505 Wsff .Zjs 0 fuxycmnezl y wn 0:];73070 3 Ja 6W7 s05 Jim-Ago 2 03 Patented Oct. 1, 1935 PATENT OFFICE CATALYTIC PRODUCTION OF SULPHUR TRIOXIDE Harry P. Bassett, Philadelphia, Pa. Application March 1, 1932, Serial No. 596,129

3 Claims.

My invention relates to the catalytic production of sulphur trioxide and to the catalyst and more particularly it relatesto a process for the conversion of sulphur dioxide from any of a number 5 of sources to sulphur trioxide, in which process a novel and highly efficient catalyst is employed. The invention further relates to a method of utilizing waste pickling liquors and to the production of sulphur trioxide therefrom.

10 One object of my invention is to provide an eflicient and expedient process for the production bf sulphur trioxide in which there is employed an active and relatively cheap catalyst which is not subject to poisoning by the contaminants accoml-B panyi'ng the sulphur dioxide to the catalytic chamber.

A further object of my invention is to furnish a process for the catalytic production of sulphur trioxide in which the removal of the dust from 20 the sulphur trioxide gas presents no problem since the catalyst is practically insoluble in the concentrated sulphuric acid of the absorption system and any that may be carried into the acid may be easily removed therefrom by simple filtra- 25 tion or. decantation without appreciable contamination of the acid.

Still another object of my invention is to providea process for the catalytic production of sulphur trioxide in which the catalyst may be recir- 30 culated through the catalytic chamber and where substantially complete conversion of the sulphur dioxide into the sulphur trioxide is obtained without accurate temperature control.

. ,Still another object of the invention is to fur- 35 nish a process for the manufacture of sulphur trioxidefrom sulphites or sulphates, particularly from ferrous sulphate which may be obtained from, waste pickling solutions, thus affording a process for the utilization of these commercial 40 wastes.

Other objects including the production of a novel catalyst will be apparent from a consideration of the specification and claims.

. In. the drawing:

45 Figure 1 illustrates diagrammatically the production of sulphur trioxide from sulphur dioxide by means of the novel catalyst;

Figure 2 shows diagrammatically the production of sulphur trioxide from sulphur dioxide obtained by the decomposition of a sulphate, specifically ferrous sulphate and Figure 3 illustrates diagrammatically one method of obtaining ferrous sulphate from waste pickling liquors.

55 My invention contemplates the use of ferric oxide (Fezos) as the catalyst in the conversion of sulphur dioxide to sulphur trioxide, which catalyst is obtained by the low temperature decomposition of an iron compound, for example an iron hydroxide or an iron oxygen-bearing salt. Any 5 one of the ferrous or ferric oxygen-bearing salts which is capable upon low temperature decomposition of forming ferric oxide may be employed, for example the sulphate, sulphite, nitrate, carbonate or acetate. For the purposes of this invention, it is to be understood that ferrous and ferric hydroxide are to be included in the phrase iron oxygen-bearing salt. In the preparation of the catalyst, the iron oxygen-bearing salt is heated at a temperature not exceeding 700 C. and preferably at a temperature in the neighborhood of 600 0., although the exact temperature to be employed will depend somewhat upon the temperature of decomposition of the particular salt being heated. Forexample, in the case of ferrous sulphate whose decomposition point is 586 0., a temperature slightly above this, in the neighborhood of 600 C., is sufficient. The heating may take place in the presence of or in the absence of air. The salt is heated for a sufiicient time to bring about the decomposition and since the time of heating is dependent upon the temperature employed and upon the mass of material to be decomposed, no definite figures can be given although I have found that in general heating for twenty minutes is all that is required. Hereinafter, throughout the specification and claims, the catalyst will be termed ferric oxide obtained by the low temperature decomposition of an iron oxygen-bearing salt.

This catalyst, which may be employed in the finely divided condition in which it is originally obtained or may be suitably agglomerated, is efficient in operation and is not subject to poisoning by the contaminants accompanyingthe sulphur dioxide. It may be successfully employed in the manufacture of sulphur trioxide from sulphur dioxide obtained from any source, for example from waste metallurgical gases, by burning sulphur, by roasting pyrites, or by heating a sulphate or a sulphite. The use of the catalyst, due to its cheapness and insolubility in the concentrated sulphuric acid of the absorption system, presents no problem in dust removal as has heretofore been encountered since any of the catalyst that is carried by the stream of sulphur trioxide into the acid absorption system may be easily removed therefrom by filtration or decantation without appreciable contamination of the acid. The catalyst is par- I ticularly adapted for continuous recirculation absorbed. as is customary in 60 to 66 Baum sulthrough the catalytic chamber and fresh supplies of catalyst may be added as desired. The continuous recirculation of the catalyst allows the process to be carried out without accurate temperature control, for if the catalyst at the end of the chamber at which the sulphur dioxide enters is at a temperature above that for correct conversion the temperature of the catalyst at some point in the chamber will be at the correct conversion temperature.

The manufacture of sulphur trioxide from sulphur dioxide gas is illustrated diagrammatically in Figure 1 where sulphur dioxide and an excess of air are passed into the catalytic chamber which may be of any suitable construction, for example, an internally or externally heated rotary furnace. While a chamber may be employed in which there is no recirculation of the catalyst, Iprefer to pass continuously the catalyst through the chamber since in this instance less accurate temperature control is required. The passage of the catalyst may either be with the flow of gases or countercurrent thereto, as. shown in the diagram. In either event, the catalyst at the end of the furnace at which the gases enter should be maintained at atemperature above 425 C-450 C. (the conversion temperature), for example at 550 C. The heat maintained in the remainder. of the furnace is immaterial so long as thetemperature at the end of the furnace at which the gases leave the chamber is below that of the conversion temperature. It will thus be seen that necessarily at some point in the chamber the catalyst is at the correct temperature for complete conversion of the sulphur dioxide into sulphur trioxide. The catalyst may be removed from the catalytic chamber and carried by a conveyor if desired to the opposite end of the chamber and again fed thereto. The sulphur trioxide gases leaving the furnace may be passed through a dust remover and then phuric acid.

As previously pointed out, the sulphur dioxide may be obtained by heating asulphate or sulphite.

In Figure 2, there is shown diagrammatically a the catalytic chamber at one. end and ferrous sulphate monohydrate crystals and the catalyst are passed into the chamber at the opposite end in contact with each other, and are continuously passed therethrough. It is at this'end of the chamber that the sulphur trioxide is removed. The catalytic chamber is maintained at a temperature at least in one portion thereof above the decomposition point of ferrous sulphate, that is to say, above 586 degrees C. Since the ferrous sulphate begins to decompose at 186 degrees C., a temperature approximating this may be maintained at the end of the chamber at which it enters, and the temperature increased to 586 degrees C., or above, as the materials continue through the chamber. Preferably, a temperature in the neighborhood of 600 degrees C. is maintained at a point near the end of the chamber at which the air enters and the catalyst leaves and in no case is the temperature in the chamber in V excess of 700 degrees C. The temperature at the -in the usual manner.

end of the chamber at which the sulphur trloxide leaves and the ferrous sulphate and catalyst enter is immaterialso long as it is below the temperature of conversion of 425 degrees C.-450 degrees 0., thus assuring a correct temperature for conver- 5 sion at some portion of the chamber. The ferrous sulphate as it is heated decomposes to form ferric oxide (FezOa) sulphur dioxide, and sulphur trioxide. The sulphur dioxide in the presence of excess 'air and in the presence of the catalyst is 10 converted into sulphur trioxide which, together with the sulphur trioxide obtained by the decomposition of the ferrous sulphate, may be freed from dust and absorbed in concentrated sulphuric acid The ferric oxide which re- 15 sults from the decomposition mingles with the ferricoxide catalyst and since itis formed by the low temperature'decomposition of an iron oxygenbearing salt, it functions as a catalyst and may be employed as such in subsequent runs. This proc- 20 ess also is particularly adapted for continuous recirculation of the catalyst, for example, in an maining portion is stored for use in subsequent or 3;

other runs or for the market as paint oxide or otherwise. The proportion of catalyst to ferrous sulphate may vary widely As a typical example, I have found that 50% of the catalyst to 50% of ferrous sulphate is satisfactory. 35

The ferrous sulphate to be treated as shown in Figure'2 may advantageously be obtained from the waste solutions resulting from the treatment of iron or steel by dilute sulphuric acid to remove the oxide film and rust therefrom. The disposal 4 of these so-called pickling solutions at the present time presentsan industrial problem since great difficulty is encountered therein due in part to legislation which prevents the pollution of streams. The volume of waste pickling solutions in the country is quite tremendous, and I propose to use these solutions as asource of ferrous sulphate for the production of sulphur trioxide.

While the ferrous sulphate may be obtained from the pickling'solutions in any suitable man- 50.;

her, for example, by evaporation of the solution, and while the heptahydrate and trihydrate are applicable as well as the monohydrate for use in the production of sulphur trioxide, I prefer to obtain the ferrous sulphate monohydrate by the 55;

method. shown diagrammatically in Figure 3. The waste pickling solution is neutralized preferably by the addition of scrap iron in amounts sufficient to counteract the acidity. The neutralized pickling solution is then passed to an evapo- (it):v

rator, shown in Figure 3 as a vacuum evaporator at I, and is continuously concentrated. When it becomes sufficiently concentrated, it deposits monohydrate from solution. The concentrated solution is circulated by'means of a centrifugal 65;

pump 2 through pipe line 3 to settling tank 4. The clear liquor is allowed to flow back to the evaporator through pipe line 8 either by vacuum or by means of a pump 9 (as shown in the drawing). The ferrous sulphate monohydrate is sepa- 7 0;-

wise, to storage to be used in the preparation of that the above process for obtaining the ferrous sulphate monohydrate is simple to carry out and a compound is obtained from the waste pickling solutions which is applicable for the production of sulphur trioxide.

Considerable modification is possibleinthe steps involved in the catalytic production of the sulphur trioxide from the sulphur dioxide, either furnished to the catalytic chamber as a gas or obtained by the decomposition of a sulphate or sulphite, as well as in the method of utilizing the waste pickling solutions without departing from the essential features of my invention.

I claim:

1. The process of obtaining sulphur trioxide from an iron oxygen-bearing salt forming sulphur dioxide and ferric oxide upon heating, as

the source of sulphur dioxide, which comprises intimately mixing said salt and a catalyst of ferric oxide obtained by the decomposition of an iron oxygen-bearing salt at a temperature not exceeding about 700 C., passing said mixture of salt and catalyst continuously through a catalytic chamber, admitting air to the chamber at the end opposite that at which the catalyst and said salt enter and the sulphur trioxide is removed, maintaining the portion of the catalytic chamber near the air entrance at a temperature above the salt decomposition temperature and the portion of the catalytic chamber near the sulphur trioxide exit at a temperature below the dissociation temperature of sulphur trioxide, the temperature 1n the chamber not exceeding approximately 700 C. and removing the sulphur trioxide gas from one end of said chamber and the ferric oxide from the opposite end thereof.

2. The process of obtaining sulphur trioxide from an iron oxygen-bearing salt forming sulphur dioxide and ferric oxide upon heating, as the source of sulphur dioxide, which comprises intimately mixing said salt and a catalyst of ferric oxide obtained by the decomposition of an iron oxygen-bearing salt at a temperature not exceeding about 700 C., passing said mixture of salt and catalyst continuously through a catalytic chamber, admitting air to the chamber at the end 5 opposite that at which the catalyst and said salt enter and the sulphur trioxide is removed, maintaining the portion of the catalytic chamber near the air entrance at a temperature above the salt decomposition temperature but less than 700 C. and the portion of the catalytic chamber near the sulphur trioxide exit at a temperature below the dissociation temperature of sulphur trioxide, the temperature in the chamber not exceeding approximately 700 C. and removing the sulphur trioxide gas from one end of said chamber and the ferric oxide from the opposite end thereof.

3. The process of obtaining sulphur trioxide from an iron oxygen-bearing salt forming sulphur dioxide and ferric oxide upon heating, as the source of sulphur dioxide, which comprises intimately mixing said salt and a catalyst of ferric oxide obtained from a previous run of said process, passing said mixture of salt and catalyst continuously through a catalytic chamber, admitting air to the chamber at the end opposite that at which the catalyst and said salt enter and the sulphur trioxide is removed, maintaining the portion of the catalytic chamber near the air entrance at a temperature above the salt decomposition temperature but less than 700 C. and the portion of the catalytic chamber near the sulphur trioxide exit at a temperature below the dissociation temperature of sulphur trioxide, the temperature in the chamber not exceeding approximately 700 C., removing the sulphur trioxide gas from one end of said chamber and the ferric oxide from the opposite end thereof, and re-using at least a portion of said ferric oxide as a catalyst in said process in admixture with further amounts of iron oxygen-bearing salt.

HARRY P. BASSETT.

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